Various electro-optical systems have been developed for reading optical indicia, such as barcodes. A barcode is a coded pattern of graphical indicia comprised of a series of bars and spaces of varying widths, the bars and spaces having differing light reflecting characteristics. Some of the more popular barcode symbologies include: Universal Product Code (UPC), typically used in retail stores sales; Data Matrix, typically used for labeling small electronic products; Code 39, primarily used in inventory tracking; and Postnet, which is used for encoding zip codes for U.S. mail. Barcodes may be one dimensional (1D), i.e., a single row of graphical indicia such as a UPC barcode or two dimensional (2D), i.e., multiple rows of graphical indicia comprising a single barcode, such as Data Matrix which comprising multiple rows and columns of black and white square modules arranged in a square or rectangular pattern.
Systems that read barcodes (i.e., barcode readers) electro-optically transform the graphic indicia into electrical signals, which are decoded into alphanumerical characters that are intended to be descriptive of the article or some characteristic thereof. The characters are then typically represented in digital form and utilized as an input to a data processing system for various end-user applications such as point-of-sale processing, inventory control and the like.
Barcode readers that read and decode barcodes employing imaging systems are typically referred to as imaging-based barcode readers or barcode scanners. Imaging systems include charge coupled device (CCD) arrays, complementary metal oxide semiconductor (CMOS) arrays, or other imaging sensor arrays having a plurality of photosensitive elements (e.g., photo-sensors) defining image pixels. An illumination system including light emitting diodes (LEDs) or other light source directs illumination light toward a target object, e.g., a target barcode. Light reflected from the target barcode is focused through a system of one or more lens of the imaging system onto the sensor array. Periodically, the pixels of the sensor array are read out to generate signal representative of a captured image frame, which can be processed by the decoding circuitry of the imaging system to decode the imaged barcode.
When an imaging scanner is used to read barcodes, it usually operates in a broad variety of ambient light conditions including bright sun or dark areas where an additional illumination system is needed. It is important to set properly the exposure time and the gain of the sensor to achieve acceptable image quality. If the exposure is too short, the image will appear dark. If the exposure is too long, the image will be for the most part over saturated. Usually it requires acquiring a few images before the optimum setting for the exposure is found.
Under low ambient light conditions, an illumination system can be used. Unfortunately the brightness of the image depends on the distance to the acquired object and its reflectivity, therefore it still may be required to acquire a few images before the optimum setting for the exposure is found.
For improving the life expectancy of the illumination system, it will be advantageous to avoid energizing the illumination system at the full brightness before the optimum setting for the exposure is found.